Friday, November 11, 2011

Sonoran Desert Rain: El Nino and La Nina

If anything is consistent in the desert, it is its consistently variable rainfall and drought.  Though still a desert, portions of the Sonoran Desert are considered some of the wettest deserts in the world.  Rainfall ranges between three and twelve inches annually, depending on location, and is split roughly between summer monsoon season and winter.  Between these "rainy" seasons there are two annual droughts, one in spring and one in fall.  Resulting from "abundant" rainfall compared to other deserts, and both warm and cool rainy seasons, the Sonoran Desert is likely the botanically richest desert in the world.  Other deserts receive less rain and only have one rainy season, if any at all.  Despite this so called abundance, rain is still sparse and variable.  I have seen years with three inches of rain and others with 20 inches.  Trace rain showers are the most common type of shower, but I have seen storms that drop eight inches, an entire years worth, in one storm over a few days.  Months often pass without even a trace of rain, then drought is broken by a storm dropping two inches in a single day, followed by more months without rain.  Variability is the rule, and much of what controls this variability is connected to ocean temperatures thousands of miles away.

Amazingly, the Pacific Ocean off the coast of South America can have huge effects on weather in the Sonoran Desert.  Within the Pacific Ocean, along the equator, water temperatures vary from season to season and year to year.  These temperatures cycle above and below the average temperatures on a seemingly an unpredictable basis.  This cycling is called the El Nino Southern Oscillation, or ENSO.  The weatherman is still trying to figure all this out but does know that both above and below ocean temperatures produce predictable weather patterns.  When equatorial ocean temperatures off the coast of South America are below average it is called a La Nina event.  When ocean temperatures are above average it is an El Nino event.  Weathermen watch for these and make long-term predictions based off of them.

El Nino, top map, with red and orange representing warmer ocean temperatures along the equator off the coast of South America.  La Nina, bottom map, with blue representing cooler ocean temperatures. 

Check out this link with lots of ENSO information, including an animation that tracks Pacific Ocean temperature change over the past three months.

Link to current National Weather Service ENSO observations: ENSO Diagnostic Discussion 

La Nina's lower ocean temperatures results in less evaporation from the ocean and therefore a weak pacific jet stream bringing moisture to the Sonoran Desert.  Typically this means winter rainfall is lower than normal.  El Nino's higher ocean temperatures results in greater evaporation from the ocean and therefore a stronger Pacific Jet Stream bringing in moisture to the Sonoran Desert.  So typically La Nina means less winter rainfall, while El Nino means more winter rainfall.  Summer rainfall patterns seem to be less controlled by ENSO.  However, La Nina does seem to occasionally increase summer rainfall but El Nino seems to have the opposite.  

Even with this information, predicting rain in the desert just about drives me crazy.  As always, even when all forecasts align, the exact opposite often happens.  Though El Nino and La Nina often do accurately predict rainfall, they also are often wrong.  The driest years I remember where always strong La Nina's.  The wettest, El Nino's.  But I have also seen wet La Nina's and below average rainfall during El Nino's.  

This winter we currently have strengthening La Nina conditions, which typically would mean less rainfall.  However, we have already had about a half inch of rain for November in the Phoenix area.  More rain is predicted in the next week or so.  This is unusual, but for the desert, the unusual is expected.  As a result of this rainfall, temperatures have been slightly below what is typical.  If even a little more rain does come in the short term, and temperatures do stay cooler, the soil will stay moist and cause an abundance of dormant wildflower seeds to germinate.  Then, if rainfall continues, say one decent rainfall every month through March, massive blooms of wildflowers will display themselves in the spring.  We will all have to wait and see, but predicting good spring desert wildflower displays is even more difficult than predicting desert rain.  
A spring desert wildflower display after a winter of above average rain.  Picture from Arizona-Sonoran Desert Museum. 

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